Save backup to (webDAV) provider (eg. Nextcloud)

[mjeveritt]

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Similar to #1354 it would be nice/useful to sync to other providers than Google. OneDrive, Dropbox are other proprietary formats, but WebDAV would allow simple synchronisation with multiple services, eg. NextCloud, and Owncloud.

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Update the 'Save Backup' option to include eg. Dropbox and WebDAV providers.
A good example plugin with a (small) variety of useful, popular providers for backup is Stylus.
There may be some useful code tips in https://github.com/openstyles/stylus/blob/master/background/sync-manager.js .

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